There was a chicken breeder who noticed that some of his birds were getting tumors and he brought them to Rous, who exposed healthy bird cells to an extract of the tumor. He was able to figure out that in this case cancer was being transmitted by a virus. Later analysis showed that the virus itself doesn't cause cancer, but that this particular strain was "lucky" and picked up an oncogene (basically took a normal tyrosine kinase gene that is involved in the cell cycle from a chicken and incorporated it into the virus, making cell replication more rapid).
This happens all the time with retroviruses in the wild. I find it interesting because the key to the virus' ability to cause tumors actually lies in using the normal cells' own machinery. Sort of like uncovering an ancient archeological site and decoding a text only to see it is about your civilization!